Life Cycles

The search for patterns in the history of life builds on the labor
of generations of paleontologists who went out in the field to dig
up fossils. It also owes a debt to one paleontologist who went into
the library to dig up thousands of records of fossil discoveries.
John Sepkoski began this research while he was still a student of
Stephen Jay Gould in the 1970s; by the time of Sepkoski's death in
1999 (at age 50), his database had grown to include more than 36,000
genera of marine organisms. The compendium was published in 2002,
both as a weighty tome and as a CD-ROM.

The Sepkoski database has a simple structure. For each genus, he
lists the oldest and the youngest geological layers in which at
least one member of the genus is reported to appear. For example,
the genus Tellinimera carries the notation "K (Camp-l)
- T (Dani), signifying that these bivalve molluscs are first
observed in the lower substage of the Campanian stage of the
Cretaceous period (which is abbreviated K to avoid confusion with
the Cambrian and the Carboniferous); the last appearances are in the
Danian stage of the Tertiary period. (Thus Tellinimera was
one of the lucky survivors of the K-T catastrophe, the extinction
that's famous for doing in the dinosaurs.)

Even though the Sepkoski compendium is available on CD-ROM, getting
it into a form suitable for further analysis is more than a routine
clerical chore. I did some preliminary reformatting with the
search-and-replace functions of a text editor, then wrote a small
program to do further processing, and finally imported the result
into a database manager. What's maddening about such a conversion
process is that even tiny typographical inconsistencies in the
text—a misplaced hyphen, an extra tab character—can
totally derail the operation. Other kinds of errors turn up, too.
For example, I found a few dozen entries where Sepkoski apparently
recorded the same genus twice. Such minor oversights are hardly a
surprise in a document that took decades to compile, and which the
author never had a chance to review and revise before publication.
In any case, for statistical purposes the database needn't be
perfect; random errors might blur a genuine periodic signal, but
they are unlikely to generate a spurious one.

The database gives the dates of fossils in terms of geologic
periods, epochs, stages and so on; for studies of periodicity, these
layers of the stratigraphic column have to be assigned dates and
durations in calendar years. As it happens, a new calibration of the
geologic sequence, assembled by the International Commission on
Stratigraphy, has just been published (in a tome even weightier than
the Sepkoski compendium). Based on radio-isotope measurements,
paleomagnetism and evidence of astronomical cycles, Geologic Time
Scale 2004, or GTS2004, gives dates for strata as far back as the
beginning of the Cambrian period—which according to GTS2004
was 542 million years ago.

The Sepkoski compendium mentions almost 300 geologic intervals, to
which Muller and Rohde assigned numerical dates based on the new
time scale. The task was not entirely straightforward because of
changes and variations in nomenclature. For example, Sepkoski refers
to a Wolf-campian epoch, which is not recognized in GTS2004; Muller
and Rohde defined it as the union of two stages.